PARIETAL EYE AND ADJACENT ORGANS IN SPHENODON. 125 



lens is very sharply marked off from the rest of the wall of the 

 vesicle. The latter, in fact, forms a thick-walled cup with a 

 sharp inturned edge, which appears to grasp the margin of the 

 lens just where the less convex outer surface of the latter joins 

 the more convex inner surface. 



Histologically the lens shows little or no change. The rest 

 of the wall of the vesicle, however^ forming what we may 

 fairly call the optic cup, has undergone marked alterations. 

 In the first place it has become distinctly differentiated into 

 two layers, an inner [In. W.), next to the cavity of the vesicle, 

 and an outer {Out. W., fig. 14). The inner layer is very much 

 thicker than the outer, and the two show a strong tendency to 

 separate from one another, except round the margin of the 

 cup, where no such separation takes place. The outer layer 

 is a single layer of rather short columnar cells with large 

 elongated oval nuclei ; it contains no pigment. The inner 

 layer shows a series of columnar cells next to the cavity of the 

 vesicle, with very faint cell boundaries and large oval nuclei, 

 while its deeper part is densely packed with large oval nuclei 

 in several tiers, between which no cell boundaries can be 

 made out. The deepest nuclei, nearest to the outer wall of 

 the cup, are smaller than the others, but except for this no 

 differences can be made out between them. The nuclei are 

 not arranged in distinctly recognisable layers. 



Small granules of black pigment, arranged in radiating lines, 

 are now conspicuous between the cells of the inner layer of the 

 wall of the optic cup, but almost confined to the inner half of 

 its thickness, and most abundant in the deeper part of that 

 half, i. e. about the middle of the inner layer. Round the 

 margin of the cup, however, the pigment is especially dense, 

 and lies more on the inner surface next to the cavity of the 

 vesicle. 



Between the outer and inner layers of the wall of the optic 

 cup, in the space formed by their slight separation, a small 

 quantity of finely granular, non-staining material is present. 

 This appears to correspond to Spencer's " molecular layer,'' 

 which in the adult parietal eye seems still to divide the retina 



